FIG. 1 is a sectional view showing the coil arrangement in a superconducting magnet which is divided into, for example, four sections. The superconducting magnet 10 shown comprises side superconducting coils 1a, 1b and center superconducting coils 2a, 2b, these superconducting coils 1a, 1b, 2a and 2b being wound in such a way that they are arranged in series. Provided around the superconducting magnet is a cylindrical ferromagnetic shield 6 of a ferromagnetic material for reducing leakage magnetic flux. Next, FIG. 2A is a connection diagram showing a conventional superconducting magnet device equipped with a superconducting magnet of the type shown in FIG. 1. In FIG. 2A, the superconducting coils 1a, 1b, 2a and 2b are sequentially connected to each other in series. Connected in parallel to these superconducting coils 1a, 1b and 2a, 2b are resistors 3a and 3b which constitute protective elements for protecting the superconducting coils. Connected to the section between the ends of this row of superconducting coils 1a, 2b, 2a and 1b which are connected in series to each other is a persistent current switch 5, thereby forming a closed circuit. The superconducting coils 1a, 1b, 2a and 2b are supplied with an electrical current from an exciting power source (not shown). After the current has reached a predetermined value, persistent current switch 5 is closed allowing a persistent current operation to be performed with the closed circuit shown in FIG. 2A.
The operation of the superconducting magnet device shown in FIG. 2A will now be described. In the superconducting magnet device in the excited state, the current in the circuit flows from the superconducting coils 1a, 1b, 2a and 2b through the closed circuit passing through persistent current switch 5, thereby retaining the persistent current condition. In this state, no current flows through the resistors 3a and 3b which are provided for protecting the superconducting coils. While an electromagnetic force is active between the superconducting coils 1a, 1b, 2a and 2b and the ferromagnetic shield 6, no unbalanced electromagnetic force is generated since these elements are arranged symmetrically.
The persistent current of the magnet generates a magnetic field that does not attenuate, keeping the magnetic field distribution constant.
On the other hand, if normal-conduction transition (quenching) trouble occurs at one of the above superconducting coils 1a, 1b, 2a and 2b, the superconducting coils 1a and 1b, which are arranged symmetrically with respect to the ferromagnetic shield 6 and the superconducting coils 2a, 2b, which are arranged in the same manner, will form current circulation circuits acting against the quenching voltage by virtue of the protective resistors 3a and 3b, respectively. This equalizes the current values of the superconducting coils arranged symmetrically with respect to the ferromagnetic shield 6, for example, the superconducting coils 1a, 1b or the superconducting coils 2a, 2b, thereby preventing the generation of an unbalanced electromagnetic force between the pair of superconducting coils 1a, 1b or between the pair of superconducting coils 2a, 2b and the ferromagnetic shield.
If quenching of the center superconducting coil 2a occurs, as shown in FIG. 2B, in a conventional device having the above-described construction, the current Ib which circulates in its initial stage through the superconducting coils 2a, 2b and the protective resistor 3b will rapidly attenuate due to a resistance 4 generated by the quenching and the protective resistor 3b, inducing an electrical current in the superconducting coils 1a, 1b which has a mutual inductance with respect to the superconducting coils 2a, 2b.
This augments the current Ia flowing through the superconducting coils 1a, 1b, so that, in the initial stage of quenching, the coil current is greater than the initial energizing current. Accordingly, the magnetic field is intensified in the vicinity of the superconducting coils 1a, 1b as compared with that during normal operation, so that an excessive electromagnetic force is active in those sections of the ferromagnetic shield 6 which are in the close vicinity of the superconducting coils 1a, 1b, i.e., in the end sections of the ferromagnetic shield. Consequently, it is necessary not only for the ferromagnetic shield to be so constructed as to be able to withstand this force but also for the superconducting coils 1a, 1b.